<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Chapter 2. Databases</title>
    <link rel="stylesheet" href="gettingStarted.css" type="text/css" />
    <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
    <link rel="start" href="index.html" title="Getting Started with Berkeley DB" />
    <link rel="up" href="index.html" title="Getting Started with Berkeley DB" />
    <link rel="prev" href="gettingit.html" title="Getting and Using DB" />
    <link rel="next" href="coredbclose.html" title="Closing Databases" />
  </head>
  <body>
    <div xmlns="" class="navheader">
      <div class="libver">
        <p>Library Version 11.2.5.3</p>
      </div>
      <table width="100%" summary="Navigation header">
        <tr>
          <th colspan="3" align="center">Chapter 2. Databases</th>
        </tr>
        <tr>
          <td width="20%" align="left"><a accesskey="p" href="gettingit.html">Prev</a> </td>
          <th width="60%" align="center"> </th>
          <td width="20%" align="right"> <a accesskey="n" href="coredbclose.html">Next</a></td>
        </tr>
      </table>
      <hr />
    </div>
    <div class="chapter" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="databases"></a>Chapter 2. Databases</h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="databases.html#DBOpen">Opening Databases</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="coredbclose.html">Closing Databases</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="DBOpenFlags.html">Database Open Flags</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="CoreDBAdmin.html">Administrative Methods</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="dbErrorReporting.html">Error Reporting Functions</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="CoreEnvUsage.html">Managing Databases in Environments</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="CoreDbCXXUsage.html">Database Example</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>In Berkeley DB, a database is a collection of <span class="emphasis"><em>records</em></span>. Records,
  in turn, consist of key/data pairings.
  </p>
      <p>
	Conceptually, you can think of a 
		
		<span>database</span> 
	as containing a two-column table where column 1 contains a key and column 2
	contains data.  Both the key and the data are managed using 
		 
		
		<code class="classname">Dbt</code>
		<span>class instances</span>
		
	(see <a class="xref" href="DBEntry.html" title="Chapter 3. Database Records">Database Records</a> for details on this 
	    <span>class</span>
	    ).
	So, fundamentally, using a DB 
		 
		<span>database</span> 
	involves putting, getting, and deleting database records, which in turns involves efficiently 
	managing information 
		<span>encapsulated by </span>
		
		
		 
		
		<code class="classname">Dbt</code>
		
		<span>objects.</span>
		
	The next several chapters of this book are dedicated to those activities.
  </p>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="DBOpen"></a>Opening Databases</h2>
            </div>
          </div>
        </div>
        <p>
        You open a database by instantiating a <code class="classname">Db</code> object
        and then calling its <code class="methodname">open()</code> method.
    </p>
        <p>
		Note that by default, DB does not create databases if they do not already exist. 
		To override this behavior, specify the 
		<a class="link" href="DBOpenFlags.html" title="Database Open Flags"><code class="literal">DB_CREATE</code></a> flag on the
		<code class="methodname">open()</code> method.
	</p>
        <p>
        The following code fragment illustrates a database open:
        
    </p>
        <a id="cxx_db1"></a>
        <pre class="programlisting">#include &lt;db_cxx.h&gt;

...

Db db(NULL, 0);               // Instantiate the Db object

u_int32_t oFlags = DB_CREATE; // Open flags;

try {
    // Open the database
    db.open(NULL,                // Transaction pointer 
            "my_db.db",          // Database file name 
            NULL,                // Optional logical database name
            DB_BTREE,            // Database access method
            oFlags,              // Open flags
            0);                  // File mode (using defaults)
// DbException is not subclassed from std::exception, so
// need to catch both of these.
} catch(DbException &amp;e) {
    // Error handling code goes here    
} catch(std::exception &amp;e) {
    // Error handling code goes here
} </pre>
      </div>
    </div>
    <div class="navfooter">
      <hr />
      <table width="100%" summary="Navigation footer">
        <tr>
          <td width="40%" align="left"><a accesskey="p" href="gettingit.html">Prev</a> </td>
          <td width="20%" align="center"> </td>
          <td width="40%" align="right"> <a accesskey="n" href="coredbclose.html">Next</a></td>
        </tr>
        <tr>
          <td width="40%" align="left" valign="top">Getting and Using DB  </td>
          <td width="20%" align="center">
            <a accesskey="h" href="index.html">Home</a>
          </td>
          <td width="40%" align="right" valign="top"> Closing Databases</td>
        </tr>
      </table>
    </div>
  </body>
</html>
